The layers that make up a semiconductor device are formed and patterned using what is called a photolithographic process in which a top photoresist layer is selectively hardened and etched by exposing it to a light pattern and etching away the portions of the photoresist that were not exposed to the light. The patterned photoresist is then used as a pattern for etching underlying layers of the semiconductor device.
In many instances, layers that overlie other layers of the semiconductor device should be patterned without that patterning extending down into the underlying layers. But in many cases the underlying layer would be affected in the same way by an etchant as the layer being etched. To prevent the etching of underlying layers, layers called etch-stop layers can be interposed between the layer to be etched and its underlying. For example, an etch-stop layer may be deposited over a metal layer in order to stop the etching of an insulating layer that overlies the metal layer. In this way, an etchant that is selective—having a substantially higher etch rate—to the overlying insulating layer relative to the etch-stop layer will be stopped or dramatically slowed upon reaching the etch-stop layer. A second etchant can then be used if desired to remove the etch-stop layer in areas exposed by the first etching process, where the second etchant can be chosen to be selective to the material used for etch-stop layer relative to the underlying metal layer. And in fact this process can continue in turn to further underlying layers.
The difficulty with using etch-stop layers is that the etch-stop layers may introduce diminished chemical adhesion in the stack of layers of the semiconductor device. This diminished chemical adhesion is particularly troublesome in applications where the device is exposed to greater mechanical stresses, such as underneath a wire-bonding pad. A wire-bonding pad introduces mechanical stresses onto the underlying semiconductor layers both during the manufacturing process and during subsequent device operation. For example, to attach a wire-bonding pad to a semiconductor device, a heated metal ball is pressed onto a contact pad on the semiconductor device. In order to form a metallic bond to the contact pad, pressure is applied between the metal ball and the contact pad. This pressure imposes uneven stresses underneath the contact pad, and the stresses and flexures can cause underlying layers to, for example, peel apart from each other.
Further, even after the metal ball is bonded to the contact pad of the semiconductor device, temperature cycling and other physical stresses on the final, packaged semiconductor device can cause relative movement between the wire-bond pad and the underlying layers of the semiconductor device. These stresses can cause the chemical adhesion between the layers to be broken and can also cause the layers to peel loose from each other. As mentioned, the bonds to the etch-stop layers may be particularly susceptible to peeling.